Heat treatment residual force refers to the final residual stress of the workpiece after heat treatment, which has a very important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause the deformation of the workpiece, and when it exceeds the strength limit of the material, the workpiece will be cracked. This is its harmful side and should be reduced and eliminated. However, under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical properties and service life of the parts, and turn harmful into beneficial. Analyzing the distribution and changing law of stress in the process of heat treatment of steel, and making it reasonably distributed has far-reaching practical significance for improving product quality. For example, the influence of reasonable distribution of surface residual compressive stress on the service life of parts has attracted extensive attention. Heat treatment stress of steel During the heating and cooling process of the workpiece, the temperature difference is formed due to the inconsistency of the cooling rate and time between the surface layer and the core, which will lead to uneven volume expansion and contraction and generate stress, that is, thermal stress. Under the action of thermal stress, since the initial temperature of the surface layer is lower than that of the core, the contraction is also greater than that of the core, so that the core is stretched. pulled. That is, under the action of thermal stress, the surface of the workpiece is finally compressed and the core is pulled. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. When the cooling rate is faster, the carbon content and alloy composition are higher, the uneven plastic deformation caused by thermal stress during the cooling process is larger, and the final residual stress is larger. On the other hand, due to the change of the structure of the steel during the heat treatment process, that is, the transformation of austenite to martensite, the increase of the specific volume will be accompanied by the expansion of the volume of the workpiece, and the various parts of the workpiece will undergo phase transformation, resulting in inconsistent volume growth and structure. stress. The final result of tissue stress changes is tensile stress on the surface and compressive stress on the core, which is just the opposite of thermal stress. The size of the structural stress is related to the cooling rate, shape, chemical composition of the material and other factors of the workpiece in the martensitic transformation zone. Practice has proved that during the heat treatment of any workpiece, as long as there is a phase change, thermal stress and tissue stress will occur. It's just that the thermal stress has already been generated before the tissue transformation, and the tissue stress is generated during the tissue transformation process. During the entire cooling process, the result of the combined effect of the thermal stress and the tissue stress is the actual stress in the workpiece. The result of the combined action of these two stresses is very complex and is affected by many factors, such as composition, shape, heat treatment process and so on. In terms of its development process, there are only two types, namely thermal stress and tissue stress. When the action directions are opposite, the two cancel each other, and when the action directions are the same, the two superimpose each other. Whether they cancel each other out or superimpose each other, the two stresses should have a dominant factor. When the thermal stress dominates, the result is that the core of the workpiece is pulled and the surface is compressed. When the tissue stress is dominant, the result is that the compressive surface of the workpiece core is in tension. The influence of heat treatment stress on quenching cracks exists in the factors (including metallurgical defects) that can cause stress concentration in different parts of the quenched parts, which can promote the generation of quenching cracks, but only in the tensile stress field (especially in the maximum under tensile stress) will appear, if there is no crack-promoting effect in the compressive stress field. The quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress, and it is also a factor that can give an important or even decisive influence to the quenching crack. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the parts in the high temperature section and make it exceed the critical quenching cooling rate of the steel to obtain the martensitic structure. As far as residual stress is concerned, this method can reduce the tensile stress on the surface of the workpiece and achieve the purpose of suppressing longitudinal cracks because it can increase the thermal stress value that offsets the effect of tissue stress. The effect will increase as the high temperature cooling rate increases. Moreover, in the case of hardenability, the larger the cross-sectional size of the workpiece, the greater the risk of cracking, although the actual cooling rate is slower. All of this is due to the fact that the thermal stress of this type of steel decreases with the increase in size, the actual cooling rate slows down, the thermal stress decreases, and the microstructure stress increases with the increase in size, and finally a tensile stress dominated by microstructure stress is formed on the workpiece. due to the characteristics of the surface. And it is very different from the traditional concept that the slower the cooling, the smaller the stress. For this type of steel, only longitudinal cracks can form in high-hardenability steels quenched under normal conditions. A reliable principle to avoid quench cracking is to try to minimize the anisochronous transformation of martensitic transformation inside and outside the section. Merely performing slow cooling in the martensitic transformation zone is not sufficient to prevent the formation of longitudinal cracks. In general, arc cracks can only be generated in non-hardenable parts. Although the overall rapid cooling is a necessary formation condition, the real reason for its formation is not in the rapid cooling (including the martensitic transformation zone) itself.Instead, the local position of the quenched part (determined by the geometric structure), the cooling rate in the high temperature critical temperature region is significantly slowed down, so there is no quenching. The transverse and longitudinal splits in large non-hardenable parts are caused by residual tensile stress with thermal stress as the main component acting on the center of the quenched part. caused by the expansion from the inside out.